1. Technical Field
The invention relates to the field of image intensifier tubes, and more particularly, to an image intensifier tube photocathode having reduced veiling glare.
2. Background Art
Image intensifier tubes are used generally in viewing devices for amplifying light and forming an image. Image intensifier tubes are known to suffer from a problem known as stray light or “veiling glare.”
There are three primary known methods for providing an aperture for the image intensifier tube: Method 1 uses a chrome mask deposited over the active cathode surface, Method 2 uses an external mask assembled on the potted tube housing or on the objective lens of the system, and Method 3 restricts the shape of the cathode window to two parallel surfaces.
Method 1 requires that a precision chrome layer be deposited over the finished cathode assembly, and this adds process time and handling. Any mismatch of the precision mask and the cathode surface will allow for reflections off the chrome surface that severely degrade veiling glare performance.
Method 2 requires that a precision “glare shield” be mounted onto the face of the image intensifier or on an element in the objective lens cell. Because an aperture needs to be very thin to avoid contributing additional reflections to the image “glare shields” are often expensive to fabricate and difficult to assemble on the respective lens element.
Method 3 requires that the cathode window be in the shape of a disc with two parallel surfaces. Because the cathode window provides mechanical as well as optical properties to the image tube this method greatly restricts the design of the device. Additionally this method is primarily for reducing veiling glare and does not provide an aperture as in the present invention.
The known designs for providing the correct aperture and reducing veiling glare use a combination of the above methods. Method 1 provides an adequate aperture for the image tube, but does not provide any glare reduction and can actually make internal reflections worse. Method 2 provides an aperture for the image tube, but only provides partial reduction to internal reflections and may actually create additional reflections at the lens interface. Method 3 only provides for a reduction in veiling glare and also limits the design options.
U.S. Pat. Nos. 4,661,079 and 4,961,025 disclose window-blackening methods. Removal of the prior blackening layer exposes the full diameter of the photocathode because of the lack of the aperture step of the present invention. Similarly, the surface of the photocathode is uniformly blackened because there is no aperture step of the present invention.
U.S. Pat. No. 6,040,657 teaches parallel window surfaces to reduce veiling glare.
While the above cited references introduce and disclose a number of noteworthy advances and technological improvements within the art, none completely fulfills the specific objectives achieved by this invention.